NATO Secretary-General Jens Stoltenberg, speaking at the Aspen Institute's annual security forum in the United States, said he was concerned at Turkey's expulsion from the F-35 programme.

NATO’s secretary-general on Wednesday praised Turkey’s role in the alliance hours after the US formally initiated the process to expel it from the F-35 fighter jet programme.

Speaking at the Aspen Institute's annual security forum in Colorado, the United States, Jens Stoltenberg said he was concerned over Turkey's expulsion.

"I am concerned about the consequences of the Turkish decision because it means Turkey will not be part of the F-35 programme," he said.

"It is not good; bad for all of us," he added.

The US announced it is removing Turkey from the F-35 stealth fighter programme, following through on threats to do so over Ankara's purchase of the Russian S-400 air defence system.

Ankara called on Washington to correct its decision, which it said will irreparably damage relations between the two countries.

"Turkey's contribution to NATO runs much deeper and much broader than the F-35. That is important," said Stoltenberg, adding the international coalition in Syria could have defeated Daesh "because of the contributions of Turkey".

"We used the bases, infrastructure and Turkey played a key part in that fight," he said, adding Turkish troops are contributing to different missions and operations, citing Kosovo and Afghanistan.

He also said that no other NATO member has raised the exclusion of Turkey from the F-35 programme "because we all see that we are dependent on each other".

Stoltenberg reiterated that the S-400 could not become part of NATO's shared missile defence but Turkey has aircraft and radars that would remain part of the system.

"The S-400, it’s not possible to integrate into the integrated NATO air defence and missile system, which is about sharing radar pictures, about joint air policing, which is about shared capabilities. And Turkey has not asked for that," Stoltenberg said.

"My responsibility is partly to try to help solve the issue. But as long as the issue is not solved, we need to minimise negative consequences," he added.

"The role of Turkey is much broader than the F-35 or S-400.”

Following protracted efforts to purchase an air defence system from the US with no success, Ankara signed a contract in 2017 to purchase the S-400s from Russia.

US President Donald Trump blamed the ongoing row over the F-35 on the Obama administration, whom he said created the problem by not allowing the Turkish government to purchase the Patriot missile system.

"As soon as they found out that they were going to have to buy the missiles, comparable missiles, not as good a missile, but a comparable almost missile from Russia - all of a sudden everybody started rushing and saying to Turkey, 'Okay, we'll sell you the Patriot missile,'" he told reporters at the White House on Tuesday.